Acute instability of the patella: is magnetic resonance imaging mandatory?

Acute dislocations of the patella represent 2 to 3% of traumatic injuries of the knee. When patients are seen in the emergency department with a dislocated patella clinical diagnosis is often very evident. However, in cases of short-lived subluxation or dislocations that have reduced spontaneously, the diagnosis can be challenging. The aim of this study was to evaluate the utility of MRI in the evaluation of acute patellar instability.

This was a prospective study of 39 cases of acute patellar instability. The study protocol included a clinical examination by an emergency medicine doctor, a standardised series of radiographs and an MRI scan within 15 days after injury.

The series included 15 female and 24 male patients aged on average 23 ± 10.5 years (11-46). Twenty-four cases were following sporting accidents and 15 cases following domestic accidents. In ten cases, a blow to the medial side of the patella led to the injury and in 29 cases, a twisting mechanism with a valgus and external rotation force was responsible. Six patients presented with a clinically dislocated patella and 31 patients were deemed to have a "swollen knee". Nineteen patients described an episode of subluxation of the patella and 14 an odd sensation within the knee without being able to specifically describe what had happened. Radiographs demonstrated trochlear dysplasia in 97.5% of cases and osteochondral lesions in 20.5% of cases. MRI scan confirmed the diagnosis of acute patellar instability in 37 cases giving a sensitivity of 95%. MRI findings included 25 MPFL lesions, 31 patella lesions of which 25 were bone oedema and six medial facet fractures, 31 lateral femoral condyle bone oedema type lesions, 11 chondral lesions of which seven were osteochondral in nature and also three acute associated ACL injuries and two MCL injuries.

MRI scan is an important tool in the diagnosis and management of acute patellar dislocation and subluxation. When symptoms are subtle, MRI facilitates a diagnosis with relative ease. MRI is particularly useful in identifying the exact nature of lesions most notably chondral lesions which are frequently of significant size and therefore often require expedited surgical treatment.